Embodiments of the present disclosure relate to a method of processing strain measurements, the method being of the type that, for example, processes phase information from two or more measured signals representing respective backscattered optical signals. Embodiments of the present disclosure also relate to an optical measurement system of the type that, for example, comprises at least two detectors arranged to generate respective measured signals in response to respective backscattered optical signals. Embodiments of the present disclosure further relate to a fibre-optic sensor apparatus of the type that, for example, comprises more than one optical fibre. Embodiments of the present disclosure also relates to a method of fibre-optic sensing, the method being of the type that, for example, comprises providing more than one optical fibre.
Hydrocarbon fluids such as oil and natural gas are obtained from a subterranean geologic formation, referred to as a reservoir, by drilling a well that penetrates the hydrocarbon-bearing formation. Once a wellbore is drilled, various forms of well completion components may be installed in order to control and enhance the efficiency of producing the various fluids from the reservoir. One piece of equipment which may be installed is a sensing system, such as a fibre optic based sensing system to monitor various downhole parameters that provide information that may be useful in controlling and enhancing production. However, wellbore applications are by no means the only applications where fibre optic sensing systems can be employed and, for example, fibre optic sensing systems find application in marine streamers.
Typically, a fibre optic sensor of the fibre optic sensing system comprises a length of optical fibre that is interrogated by launching pulses of light into the optical fibre. To measure vibration or strain, distributed fibre optic sensing systems measure, for example, the amplitude of Rayleigh backscatter returned from the fibre optic sensor when excited by the pulses of light. Such sensing systems are useful for tracking the movement of certain events and/or classifying various types of disturbances. However, for some applications, phase-related measurements can be used to determine other parameters.
One known fibre optic sensing system is a Distributed Vibration Sensor (DVS) system, for example a heterodyne DVS (hDVS) system. In such a sensing system, dynamic range is an important system parameter. In this respect, the dynamic range of a system interrogating a single fibre is sometimes insufficient for some applications, for example active and passive seismic measurement. In particular, a single fibre having a certain amplitude sensitivity can be inadequate where the fibre is exposed to acoustic waves of different amplitudes.
Due to the optical fibre having a single amplitude sensitivity, where a mixture of acoustic signals of different amplitudes are incident upon the optical fibre at a given location on the fibre, the acoustic signals of higher amplitude can cause signal saturation in the processing of phase data generated by acquisition circuitry of an hDVS unit in response to the mixture of acoustic signals sensed by the optical fibre. Consequently, it is not possible to apply certain processing techniques to the phase data generated by the hDVS unit in order to reconstruct a time-varying strain signal associated with a location along the optical fibre. For example, it is not possible to reconstruct the strain signal from using a so-called “phase unwrapping” processing technique, because the strain signal is aliased in the phase domain as a result of the influence of the rate of change of the acoustic signals. The aliasing is caused by the optical signal being sensed having a phase variation that is wrapped multiple times, the number of times being indeterminable by the hDVS unit. The rate of change of a signal, for example the phase of the optical signal, depends on its amplitude and frequency. For signals with the same frequency or with comparable frequency component content, the rate of change is primarily affected by their amplitude.
United States Patent Publication No. 2013/0291643 discloses a directionally sensitive Distributed Acoustic Sensing (DAS) fibre optical assembly comprising a pair of optical fibres, each having different directional acoustic sensitivities so that directions of acoustic signals relative to the optical fibres can be detected. However, this document does not address the problem of signal aliasing in the phase domain.